This invention relates to metal working operations such as turning, milling, facing, threading, boring and grooving, and, more particularly, to a method and apparatus for performing such metal working operations at high speeds with extended insert life.
Most machining operations are performed by a cutting tool which includes a holder and one or more cutting inserts each having a top surface terminating with one or more cutting edges. The tool holder is formed with a socket within which the cutting inserts are clamped in place. The leading or cutting edge of an insert makes contact with the workpiece to remove material therefrom in the form of chips. A chip comprises a plurality of thin, generally rectangular-shaped sections of material which slide relative to one another along shear planes as they are separated by the insert from the workpiece. This shearing movement of the thin sections of material relative to one another in forming a chip generates a substantial amount of heat, which, when combined with the heat produced by engagement of the cutting edge of the insert with the workpiece can amount to 1500.degree.-2000.degree. F.
Among the causes of failure of the cutting inserts of tool holders employed in prior art machining operations are abrasion between the cutting insert and workpiece, and a problem known as cratering. Cratering results from the intense heat developed in the formation of the chips and the frictional engagement of the chips with the cutting insert.
As the material forming the chip is sheared from the workpiece, it moves along at least a portion of the exposed top surface of the insert. Due to such frictional engagement, and the intense heat generate in the formation of the chip material along the top portion of the insert is removed forming "craters". If these craters become deep enough, the entire insert is subject to cracking and failure along its cutting edge, and along the sides of the insert, upon contact with the workpiece. Cratering has become a particular problem in recent years due to the development and extensive use of hard alloy steels, high strength plastics and composite materials formed of high tensile strength fibers coated with a rigid matrix material such as epoxy.
Prior attempts to avoid cratering and wear of the insert due to abrasion with the workpiece have provided only modest increases in tool life and efficiency. One approaching the prior art has been to form inserts of high strength materials such as tungsten carbide. Although extremely hard, tungsten carbide inserts are brittle and are subject to chipping which results in premature failure. To improve the lubricity of inserts, such materials as hardened or alloyed ceramics have been employed in the fabrication of cutting inserts. Additionally, a variety of low friction coatings have been developed for cutting inserts to reduce the friction between the cutting insert and workpiece.
In addition to the improved materials and coatings used in the manufacture of cutting inserts, attempts have been made to increase tool life by reducing the temperature in the "cutting area", i.e., the cutting edge of the insert, the insert-workpiece interface and the area on the workpiece where material is sheared to form chips.
One method of cooling practiced in the prior art is flood cooling which involves the spraying of a low pressure stream of coolant toward the cutting area. Typically, a nozzle disposed several inches above the cutting tool and workpiece directs a low pressure stream of coolant toward the workpiece, tool holder, cutting insert and on top of the chips being produced.
The primary problem with flood cooling is that it is ineffective in actually reaching the cutting area. The underside of the chip which makes contact with the exposed top surface of the cutting insert, the cutting edge of the insert and the area where material is sheared from the workpiece, are not cooled by a low pressure stream of coolant directed from above the tool holder and onto the top surface of the chips. This is because the heat in the cutting area is so intense that a heat barrier is produced which vaporizes the coolant well before it can flow near the cutting edge of the insert.
Several attempts have been made in the prior art to improve upon the flood cooling technique described above. For example, the discharge orifice of the nozzle carrying the coolant was placed closer to the insert and workpiece, and/or fabricated as an integral portion of the tool holder, to eject the coolant more directly at the cutting area. See, for example, U.S. Pat. Nos. 1,695,955; 3,323,195; and 3,364,800. In addition to positioning the nozzle nearer to the insert and workpiece, the stream of coolant was ejected at higher pressures than typical flood cooling applications in an effort to break through the heat barrier developed in the cutting area. See U.S. Pat. No. 2,653,517.
Other tool holders for various types of cutting operations were designed to incorporate coolant delivery passageways which direct the coolant flow across the exposed top surface of the insert toward the cutting edge in contact with the workpiece. In these designs, a separate conduit or nozzle for spraying the coolant toward the cutting area was eliminated making the cutting tool more compact. Examples of this type of design are shown in U.S. Pat. Nos. 4,302,135; 4,072,438; 3,176,330; 3,002,140; 2,360,385; and, West German Patent No. 3,004,166.
A common problem with the apparatus disclosed in the patents mentioned above is that coolant in the form of an oil-water or synthetic mixture, at ambient temperature, is directed across the top surface of the insert toward the cutting area without sufficient velocity to pierce the heat barrier surrounding the cutting area. As a result, the coolant failed to reach the interface between the cutting insert and workpiece and/or the area on the workpiece where the chips are being formed before becoming vaporized. Under these circumstances, no heat was dissipated from the cutting area to prevent cratering. In addition, this failure to remove heat from the cutting area created a significant temperature differential between the cutting edge of the insert which remained hot, and the rear portion of the insert which was cooled by coolant, causing thermal failure of the insert.
The failure in the prior art to effectively reduce temperature in the cutting area results in a number of disadvantages and limitations in machining operations. As discussed above, high temperatures cause insert failure. This directly affects production speed in several ways. In order to reduce temperatures, the machine tools must be run at lower speeds and at reduced depths of cut and feed rates which lowers productivity. If speeds are increased, the downtime of the machine tool increases because the inserts must be replaced more frequently. The less time the insert is in the cut, the lower the productivity of a given machine tool. Overall productivity is therefore limited by the useful life and performance of the cutting inserts which have historically lagged far behind the operating speeds of machine tools.
Another serious problem in present day machining operations involves the breakage and removal of chips from the area of the cutting insert, tool holder an the chucks which mount the workpiece and tool holder. If chips are formed in continuous lengths, they tend to wrap around the tool holder or chucks which almost always leads to tool failure or at least requires a periodic interruption of the machining operation to clear the area of impacted or bundled chips. This is particularly disadvantageous in flexible manufacturing systems in which the entire machining operation iis intended to be completely automated. Flexible manufacturing systems are designed to operate without human assistance and it substantially limits their efficiency if a worker must regularly clear impacted or bundled chips.
Current attempts to solve the problem of removal and breakage of chips are limited to various designs of chip breaker grooves in cutting inserts. Chip breaker grooves extend inwardly from the exposed top surface of the insert, and are spaced from the cutting edge. The chip breaker grooves engage the chips as they are sheared by the cutting edge from the workpiece, and then turn or bend them upwardly from the exposed surface of the insert so that the chips tend to fracture.
While acceptable performance has been achieved with some chip breaker groove designs in some applications, variables in machining operations such as differing materials, types of machines, depths of cut, feed rates and speeds make it virtually impossible for one chip breaker groove design to be effective in all applications. This is evidenced by the multitude of chip breaker designs now available. Selection of a suitable cutting insert for a particular machining application, if one exists at all, is a difficult and continuing problem.